High-frequency power transistors have traditionally occupied a large amount of the area available on a monolithic integrated circuit. The need for high output power generally requires that the output stage of a transistor amplifier have a very large gate or emitter periphery. The gates of MESFETs or the emitters of heterojunction bipolar transistors (HBTs) often exceed one millimeter in total periphery, and can extend across the entire width of the integrated circuit. See, for example, M. Avasarala et al., "A 2.5-Watt High Efficiency X-Band Power MMIC," IEEE Microwave and Millimeter-wave Monolithic Circuits Symposium Digest of Papers, 1989, pp. 25-28, and J. J. Komiak, "Octave Band Eleven Watt Power Amplifier MMIC," IEEE Microwave and Millimeter-wave Monolithic Circuits Symposium Digest of Papers, 1990, pp. 35-38.
An additional difficulty is that for high-frequency applications, one generally must ensure that the phase velocity along all signal paths to the power transistor is relatively constant; otherwise, signals amplified by different portions of the transistor may partially cancel one another and result in diminished output power. FIG. 1 shows a typical prior art power transistor 18 as may be used in the output stage of a power amplifier. A signal incident at input port 20 is distributed by a microstrip transmission line network 22 to the input terminals 24 (in this case the gate terminal of a MESFET) of transistor unit cells 26. The amplified signals exit the output terminals 28 (i.e. the drain terminal) of transistor unit cells 26. The signals from each of the unit cells 26 of transistor 18 are combined by a microstrip transmission line network 30, and may be extracted at the output port 32. This arrangement ensures that the phase velocity along each of the paths to and from transistor unit cells 26 is equal. The power combined at the output of the transmission line network 30 is in phase and represents the maximum power obtainable from the transistor 18 for a given bias condition and frequency. This arrangement is effective for equalizing phase velocity, but it occupies a large amount of space on an integrated circuit. The need for large transistors and extensive phase equalization circuitry combine to make high-frequency power transistor amplifiers costly. There is a need in the industry for reducing the size and the associated costs of these devices.
Closely related to these problems is that of the physical layout and size of very large transistors. Center-fed transistors, also known as the "pi (.pi.) configuration," are known to suffer from phase differences along the central input line. This is the so-called "distributed effect," and applies to the effects of phase along the center-feed line, whether it be a gate feed or a base feed. The transistor shown in FIG. 11 is an example of a typical center-feed transistor, in this case a bipolar transistor. See also B. Bayraktaroglu et al., "5 W Monolithic HBT Amplifier for Broadband X-band Applications," IEEE Microwave and Millimeter-wave Monolithic Circuits Symposium Digest of Papers, 1990, pp. 43-46. In FIG. 11, the signal to be amplified is incident at base pad 120 and propagates along the input transmission line 122. The signal enters the transistor along the base fingers 124. In this example, the emitter of the transistor is coupled to ground through the backside of the substrate at the emitter pad 128, which in turn is coupled to the unit transistors by the emitter fingers 126. The amplified signal exits the unit transistors along the collector bus 130 which forms an output transmission line. The amplified output signal is then available at the output terminal 132.
This prior art arrangement performs well unless the size of the transistor is increased appreciably by adding more unit transistors along a longer input transmission line 122. As long as the transistor is much smaller than the wavelength of the signal, the phase of the signal at each of the unit transistors that comprise the overall transistor is approximately the same. However, if the size of the transistor (i.e. the length along the input transmission line) approaches approximately one-sixteenth of the guided wavelength of the signal being amplified, the phase progression of the signal along the input transmission line adversely affects the output power of the transistor. This phase limitation is an obstacle to the design of large transistors which provide high power levels at microwave frequencies. Aspects of the invention are intended to address these problems.